Resonant photoelastic modulation for time-of-flight imaging with standard image sensors

Imaging has played a pivotal role throughout human history, with various optical imaging systems invented over the last several centuries. Some of these inventions include the pinhole camera, the photographic film, charge coupled device sensors, and most recently CMOS image sensors. CMOS image sensors are the dominant image sensor in the present day, offering high performance at a low cost. Standard CMOS image sensors are designed to mimic human vision; they are 2D sensors capable of capturing the brightness and color per pixel. These sensors were primarily designed for humans to capture images, but there is a fundamental change recently in users of imaging systems. Significant developments in robotics and artificial intelligence in the last decade have enabled intelligent autonomous platforms. These platforms, similar to humans, require sophisticated imaging systems to interact effectively with their environment. The existing standard image sensors (2D sensors) are not the most effective sensors for these platforms; depth (3D) sensing provides significant value. Unfortunately, existing standard CMOS image sensors are unable to measure the depth dimension. Although custom-built 3D sensors are able to measure the depth axis, it would be a significant advance to adapt the already mature and high-performance standard image sensors to function as high-resolution depth sensors. In this work, we describe a method to convert a standard CMOS image sensor into a depth sensor by integrating it with a fundamentally new kind of free-space intensity modulator. We explain the design, fabrication, and working principle of this new free-space intensity modulator that we refer to as a longitudinal piezoelectric resonant photoelastic modulator. The optical modulator described in this work uses a simple fabrication process while exhibiting record-setting energy efficiency among resonant free-space intensity modulators operating in the megahertz frequency regime. The simple fabrication process and low-drive power required to operate the modulator allow for easy integration with any standard image sensor. We demonstrate proof-of-concept time-of-flight imaging experiments by integrating the fabricated modulator with a standard CMOS image sensor